<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>结果</div>
<script>

  let i1=document.querySelector("#i1")
  let i2=document.querySelector("#i2");
  let d=document.querySelector("div");

  function f(x){
      if(isNaN(i1.value)||isNaN(i2.value)){
          d.innerText="输入错误!";
          return;
      }
      switch (x){
          case 1:
              d.innerText="结果:"+(parseFloat(i1.value)+parseFloat(i2.value));
              break;
          case 2:
              d.innerText="结果:"+(parseInt(i1.value)-parseInt(i2.value));
              break;
          case 3:
              d.innerText="结果:"+(parseInt(i1.value)*parseInt(i2.value));
              break;
          case 4:
              d.innerText="结果:"+(parseInt(i1.value)/parseInt(i2.value));
              break
      }
  }

</script>
</body>
</html>